A conventional point to point radio system is shown in FIG. 1. Typically transmission is between antennas mounted on two towers, Tower 1 and Tower 2. Each tower has two antennas mounted one above the other. The upper antenna is used to transmit and receive signals, but the lower antenna is conventionally used for receive only. The purpose of the lower receive antenna is to provide diversity, i.e. an alternative signal path which may be selected should the link between the two upper antennas fade due to the propagation characteristics of the atmosphere.
The height above the ground of the upper antenna is selected to minimize obstructions in the first Fresnel zone (referred to hereafter as the Fresnel zone). The Fresnel zone is the region in which obstructions may cause signal loss. Signals traveling from one antenna to the other within the Fresnel zone will arrive with a phase difference between 0 and 90 degrees relative to the line of sight phase, unless reflections from obstructions occur. Typically 20% of the Fresnel zone may be obstructed without significant signal loss. As shown in FIG. 1, the curvature of the earth's surface needs to be taken into account when assessing the intrusion of obstructions into the Fresnel zone. Due to the variation of the density of the atmosphere with elevation, and hence variation of the dielectric constant, radio waves will tend to follow a curved path, so that the apparent curvature of the earth will vary with atmospheric conditions. The apparent radius of the earth typically has a value of 4/3 r for a 50 km link, where r is the geometric radius of the earth. However the apparent radius can fall to as little as 0.8 r for a small percentage of the time, which has the effect of increasing the apparent height of obstructions. Typically the height of the upper antennas is chosen so that obstructions will not cause significant loss in any expected atmospheric conditions.
The height above the ground of the second antenna is a tradeoff between the need to provide sufficient spacing from the upper antenna to provide diversity and the minimization of obstructions to the Fresnel zone. Typically, the lower antenna is sited such that the obstruction of the Fresnel zone is not sufficient to cause significant loss in normal atmospheric conditions, but some signal loss may be experienced when propagation is unfavorable.
The path between the two lower antennas will typically experience significant obstructions to the Fresnel zone in normal atmospheric conditions, as shown in FIG. 2, so conventionally the lower antenna is not used for transmission.